Earthquake & Seismic Zones — Set 2

Correct Answer: B. Charles Francis Richter

The Richter Scale was developed by American seismologist Charles Francis Richter in 1935 at the California Institute of Technology. It was originally designed to measure the magnitude of earthquakes in California using seismograph readings. The scale is logarithmic, meaning each whole number increase represents a 10-fold increase in measured wave amplitude and approximately 31.6 times more energy release.

Correct Answer: B. Mw is more accurate for large earthquakes while Richter is better for local small earthquakes

The Moment Magnitude Scale (Mw) is more accurate for measuring large earthquakes (above magnitude 6.5) while the Richter Scale (ML) was originally designed for local small to moderate earthquakes in California. The Mw scale is based on seismic moment which directly relates to the energy released, making it more physically meaningful. Modern seismologists primarily use Mw, and most large earthquake magnitudes reported today use this scale.

Correct Answer: B. The underground point where the earthquake originates

The focus (or hypocentre) of an earthquake is the underground point where the rupture first begins and seismic energy is released. This is the actual origin point of the earthquake within the Earth's crust or mantle. The depth of the focus significantly affects the damage pattern — shallow focus earthquakes (less than 70 km) generally cause more surface damage than deep focus earthquakes.

Correct Answer: B. The point on Earth's surface directly above the focus

The epicentre of an earthquake is the point on Earth's surface directly above the focus (hypocentre). It is typically the location of maximum ground shaking and is the point from which earthquake distances are measured. News reports typically state the earthquake occurred 'near' or 'at' the epicentre location when reporting seismic events.

Correct Answer: B. Longitudinal compression waves that travel fastest

P waves (Primary waves) are longitudinal compression waves — they compress and expand the rock in the direction of wave travel, similar to sound waves. They are the fastest seismic waves and arrive first at seismograph stations after an earthquake. P waves can travel through solids, liquids, and gases, making them useful for studying the Earth's interior structure.

Correct Answer: B. Transverse shear waves that shake ground perpendicular to wave travel

S waves (Secondary waves) are transverse shear waves that shake the ground perpendicular to the direction of wave travel, similar to shaking a rope sideways. They are slower than P waves and arrive second at seismograph stations. S waves cannot travel through liquids, which is why they are absent in the Earth's liquid outer core — this property helped scientists determine the Earth's internal structure.

Correct Answer: B. Most of the ground shaking and building damage near the epicentre

Surface waves (Love waves and Rayleigh waves) travel along Earth's surface and are responsible for most of the ground shaking and building damage near the earthquake epicentre. Although slower than body waves (P and S waves), surface waves have larger amplitudes and carry most of the destructive energy felt during an earthquake. Their amplitude decreases with depth, making surface structures most vulnerable.

Correct Answer: B. Sandy soil behaving like liquid when saturated with water and shaken by earthquake

Liquefaction occurs when water-saturated sandy soil temporarily loses its strength and behaves like a liquid due to the increased water pressure caused by earthquake shaking. Buildings and other structures may sink, tilt, or collapse when the underlying soil liquefies. Liquefaction was a major cause of damage during the 1934 Bihar-Nepal earthquake in the Gangetic Plains and is a significant hazard in India's river valleys.

Correct Answer: C. Thrust fault (reverse fault — compressional)

Thrust faults (reverse faults), where one crustal block moves up and over another due to compressional forces, are associated with subduction zones and generate the largest tsunamis. The 2004 Indian Ocean Tsunami was generated by a megathrust earthquake at the Sunda subduction zone where the Indian plate subducts under the Burmese plate. These earthquakes cause large vertical displacement of the seafloor, which displaces a massive volume of water to create tsunamis.

Correct Answer: B. Earthquake-resistant design of structures

IS 1893 is the Bureau of Indian Standards (BIS) code for earthquake-resistant design of structures in India. It provides the criteria for earthquake-resistant design including seismic zone map, design spectra, and design forces for different types of structures. The code is mandatory for all new constructions in earthquake-prone areas and is regularly updated to incorporate new research and lessons from major earthquakes.